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Solar System Moons Series

Ganymede

The largest moon in the solar system —bigger than Mercury— and the only one with its own magnetic field.

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Ganymede (NASA/JPL, public domain)
Ganymede (Galileo spacecraft, NASA/JPL, public domain). Not to scale.
5,262.4 km
Diameter
1,070,000 km
Distance from Jupiter
7.155 days
Orbital period
~110 K
Temperature

Ganymede orbits Jupiter at 1,070,000 km and, at 5,262.4 km across, is larger than the planet Mercury. It is a differentiated world: iron core, rocky mantle, and a thick shell of ice and water.

Galileo discovered in 1996 that it generates its own magnetic field —the only moon that does— and Hubble found hints of a salty ocean under the ice by watching its auroras. ESA's JUICE mission will study it up close in the coming decade.

Measured data [DATO]

DataValueStatusSource
Diameter5,262.4 km (the largest moon)[DATO] observadoNASA Planetary Fact Sheets
Mass1.4819×10²³ kg[DATO] observadoNASA Planetary Fact Sheets
Mean density1,940 kg/m³[DATO] observadoNASA Planetary Fact Sheets
Surface gravity1.428 m/s²[DATO] observadoNASA Planetary Fact Sheets
Orbital period7.155 days[DATO] observadoNASA Planetary Fact Sheets
Rotation periodsynchronous (7.155 days)[DATO] observadoNASA Planetary Fact Sheets
Distance from Jupiter1,070,000 km[DATO] observadoNASA Planetary Fact Sheets
Temperature~110 K[DATO] observadoNASA Science
AtmosphereTenuous: oxygen[DATO] observadoNASA Science
Notable featureOnly moon with its own magnetic field (Galileo, 1996)[DATO] observadoNASA Science

Truths [DATO]

  • [DATO] Ganymede is the largest moon in the solar system (5,262.4 km): bigger than Mercury (4,879 km) and Pluto.
  • [DATO] It is the only moon with its own magnetosphere, generated by a convecting iron core — like Earth, in miniature.
  • [DATO] Its auroras, observed by Hubble, wobble less than expected: the explanation is a salty ocean under the ice damping the motion.
  • [DATO] Its surface mixes ancient dark terrain and younger bright grooves: two geological ages in one world.

Labeled myths [MITO]

[MITO] Ganymede should be a planet because it is bigger than Mercury.

False: the planet definition (IAU, 2006) requires orbiting the Sun; Ganymede orbits Jupiter, so it is a moon by definition, regardless of size.

Fuente: NASA Science

[MITO] Ganymede is just an inert ball of ice.

False: it has an iron core, its own magnetic field, a likely inner ocean and a storied geology — it is one of the most complex worlds in the solar system.

Fuente: NASA Science

Open questions [PREGUNTA]

  • [PREGUNTA] How many alternating layers of ice and ocean lie inside?
  • [PREGUNTA] How does it generate and sustain its magnetic field?
  • [PREGUNTA] What will JUICE report when it arrives in the coming decade?

The chain in brief

  1. [DATA] 1. Bigger than a planet: At 5,262.4 km across, it is the largest moon in the solar system: larger than the planet Mercury (4,879 km) and than Pluto.
  2. [DATA] 2. A differentiated world: It orbits Jupiter at 1,070,000 km, and its interior is separated into layers: iron core, rocky mantle, and a thick shell of ice and water.
  3. [DATA] 3. The only moon with its own magnetic field: Galileo discovered in 1996 that Ganymede generates its own magnetosphere, with a convecting iron core — like Earth, in miniature.
  4. [MODEL] 4. Auroras that betray an ocean: Hubble observed that its auroras wobble less than expected: the explanation is a salty ocean under the ice damping that motion.
  5. [DISPUTED] 5. Layers yet to count: How many alternating layers of ice and ocean lie inside, and how it generates and sustains its magnetic field, remain open; ESA's JUICE mission will study it up close in the coming decade.